1. Field of the Invention
The present invention relates to a thin film electroluminescent (called hereinafter EL) device which is excellent in quality of display and expected to provide a multicolor EL panel as well as a full-color EL panel which is excellent in the space factor when used as a planar EL display. More particularly, the present invention relates to a thin film EL device including a luminescent layer capable of emitting luminescence of high brightness.
2. Description of the Prior Art
A thin film EL device includes an insulating layer and a luminescent layer sandwiched between a transparent electrode and a back electrode. The principle of emission of luminescence from such an EL device is generally explained as follows. In response to the application of an alternating electric field having a strength of about 10.sup.6 V/cm across the transparent electrode and the back electrode, electrons are injected into the conduction band of the luminescent layer from the interface between the insulating layer and the luminescent layer. The injected electrons are then accelerated by the applied electric field to gain energy high enough to impinge and excite luminescent centers in the luminescent layer, and electroluminescence occurs when the excited luminescent centers return to the ground state.
A thin film EL device having a so-called double insulating layer structure is known, in which a luminescent layer containing zinc sulfide (ZnS) as a host material and having manganese (Mn) added to act as luminescent centers is sandwiched between a pair of insulating layers, and the insulating layers are further sandwiched between a pair of electrodes at least one of which is transparent. Such a thin film EL device is featured by a luminescence of high brightness and an extended useful service life and is now commercially available as an EL display having a light weight and a thin thickness. This thin film EL display emits a yellowish orange luminescence when manganese (Mn) is added to zinc sulfide (ZnS) as described above. When other elements such as thulium (Tm), samarium (Sm) and terbium (Tb) are added to the ZnS, the thin film EL display emits luminescences of different colors which are blue, red and green respectively. However, in the case of the ZnS, brightness of luminescences of these three primary colors is still insufficient.
Emission of multiple colors has been strongly demanded for an EL display, and researches and studies have been made so as to find a suitable host material preferably combined with elements such as those described above. Addition of a very small amount of cerium (Ce) to strontium sulfide (SrS) provides an EL layer emitting a bluish green luminescence, addition of a very small amount of europium (Eu) to calcium sulfide (CaS) provides an EL layer emitting a red luminescence, and addition of a very small amount of terbium (Tb) to zinc sulfide (ZnS) provides an EL layer emitting a green luminescence. Use of calcium sulfide (CaS) as a host material of an EL layer is disclosed in Japanese Patent Laid-Open Publication No. 224292/86. According to the disclosure of the publication, an EL device emitting luminescence of high brightness is obtained when the crystals of its thin film EL layer tend to have a (222) orientation.
Also, "Society for Information Display 85" Digest, No. 219, pp. 218-221 reports that, when strontium sulfide (SrS) is used as a host material of a luminescent layer, its crystals especially tend to have a (111) orientation strongly.
A prior art thin film of strontium sulfide (SrS) has been deposited by evaporation at a relatively low deposition rate of about 5.ANG./s in order that the film can be successfully formed.
However, a thin film EL device using a prior art strontium sulfide film as described above has had a problem of emission of luminescence of low brightness. According to the results of investigation made by the inventors, this problem of emission of luminescence of low brightness is attributable to the fact that electrons tend to be scattered or dispersed and are not efficiently accelerated because of the presence of many crystal defects, lattice distortions and other defects occurred in the luminescent layer during the formation of the strontium sulfide film, and the brightness of luminescence is thereby lowered.
In order to increase the brightness of luminescence emitted from a thin film EL device, it becomes necessary to sufficiently clarify the physical and other factors including the crystallinity of its luminescent layer, the orientation of the crystals and the property of the interface between the luminescent layer and the adjoining insulating layer so as to make clear the principle of luminescence. However, most of these factors have not been clarified yet and are still unknown, and a thin film EL device capable of emitting luminescence of sufficiently high brightness has not been available up to now.